COMMENT: The Douglas A-26 “Invader” was Douglas Aircraft’s successor to the A-20 “Havoc”, in British service known as Douglas “Boston”, and was one of the most successful and widely operated types flown by Allied air forces in World War II. It was a twin-engine light bomber and ground attack aircraft, was fast and capable of carrying twice its specified bomb load.
A re-designation of the type from A-26 to B-26 led to confusion with the Martin B-26 “Marauder”, which first flew in November 1940, about 16 months before the Douglas design’s maiden flight. Although both types were powered by the widely used Pratt & Whitney R-2800 “Double Wasp” eighteen-cylinder, double-row radial engine, they were completely different and separate designs. Roughly 5,300 Martin “Marauders”, originated in 1939, were produced twice as many in comparison to the Douglas design.
The Douglas XA-26 prototype first flew on July 1942. Flight tests revealed excellent performance and handling, but problems with engine cooling led to cowling changes and elimination of the propeller spinners on production aircraft. Repeated collapses during testing led to strengthening of the nose landing gear. The Douglas A-26 was originally built in two different configurations. The A-26B had a gun nose housed six to eight .50 caliber machine guns, officially termed the “all-purpose nose”, later commonly known as the “six-gun nose” or “eight-gun nose”. The A-26C’s “glass” nose, officially termed the “Bombardier nose”, contained a Norden bombsight for medium altitude precision bombing. The A-26C nose section included two fixed M-2 guns, later replaced by underwing gun packs or internal guns in the wings.
After about 1,570 production aircraft, three guns were installed in each wing, coinciding with the introduction of the “eight-gun nose” for A-26Bs, giving some configurations as many as 14 .50 in machine guns in a fixed forward mount. The A-26C nose section could be exchanged for an A-26B nose section, or vice versa, in a few man-hours, thus physically changing the designation and operational role. The “flat-topped” canopy was changed in late 1944 after about 820 production aircraft, to a clamshell style with greatly improved visibility. Alongside the pilot in an A-26B, a crew member typically served as navigator and gun loader for the pilot-operated nose guns. In an A-26C, that crew member served as navigator and bombardier, and relocated to the nose section for the bombing phase of an operation In most missions, a third crew member in the rear gunner’s compartment operated the remotely controlled dorsal and ventral gun turrets, with access to and from the cockpit possible via the bomb bay only when that was empty (Ref.: 24).

COMMENT: In 1936 the Bell Aircraft Corporation’s design team began work on the Bell XP-39, a radical design of a single-seat fighter with the engine mounted behind the pilot, driving the airscrew by means of an extension shaft. This arrangement appeared to offer superior manoeuvrability, the engine weight being concentrated around the fighter’s center of gravity. But the first flight test proved that this unorthodox fighter had a low ceiling, slow rate of climb and relative lack of manoeuvrability. So alternatively the engine was mounted forward and the cockpit was positioned to the back. This and some more minor changes led to the design of the Model 3. But calculations proved no advantage of this model compared to the P-39 “Aircrobra”, so the project was not further followed (Ref.: 13).

POWER PLANT: One Pratt and Whitney R-2800-21 radial engine, rated at 2,300 hp

PERFORMANCE: 433 mph at 30.000 ft

COMMENT: The Republic P-47 D ‘Thunderbolt’ differed little from its predecessor P-47 C apart from changes in the turbo-supercharger exhaust system, water injection as standard for the R-2800-21 engine, and some minor changes. The P-47 D was the first version of the ‘Thunderbolt’ to serve with the USAAF in the pacific theatre. Towards the end of 1943, 8th Air Force ‘Thunderbolts’ began returning from escort missions “on the deck”, strafing targets of opportunity with their unused ammunition, and their success was partly responsible for the adaptation of the ‘Thunderbolt’ for what was to become its most successful role – that of a fighter-bomber. More than 5,800 P-47D ‘Thunderbolts’ are built, all possessed the original framed sliding canopy introduced on the initial production B-model. Later versions were equipped with an all-round vision bubble-type cockpit canopy (Ref.: 24)

COMMENT: The main role for the Republic P-47N was as an escort fighter for the Boeing B-29 Superfortress bombers flying raids on the Japanese home islands. But in the final stage of the war the P-47N was used very successfully as a ground attacker. App. 1.100 P-47N’s were equipped with zero-length rocket launchers for six or 10 rockets, depending on whether or not bombs or drop tanks were carried under the wings. In April 1945, the 318 Fighter Group was re-equipped on P-47N’s and operated from Ie Shima island, off the coast of Okinawa. Bombing and strafing missions were flown until the end of the hostilities (Ref.: 9).

COMMENT: The Bell P-39 “Airacobra” was one of the main American fighter aircraft in service when the United States of America entered the World War II. Designed by Bell Aircraft, it had an innovative layout, with the engine installed in the center fuselage, behind the pilot, and driving a tractor propeller via a long shaft. It was also the first fighter fitted with a tricycle undercarriage. Although its mid-engine placement was innovative, the P-39 design was handicapped by the absence of an efficient turbo-supercharger, limiting it to low-altitude work. The XP-39 made its maiden flight on 6 April 1938 achieving 390 mph at 20,000 ft, reaching this altitude in only five minutes. A production order was placed and by the time of the Pearl Harbor attack, nearly 600 P-39s had been built. When P-39 production ended in August 1944, Bell had built 9,558. Most important variants were Bell P-39N and P-30Q, if which 4,773 have been built. The “Airacobra” saw combat throughout the world, particularly in the Southwest Pacific, Mediterranean and Russian theaters. But the “Airacobra” found itself outclassed as an interceptor and the type was gradually relegated to other duties. It often was used at lower altitudes for such missions as ground strafing (Ref.: 23).

COMMENT: While the production of the Republic P-47 Thunderbolt had been emerging in ever-mounting quantities, Republic’s engineers had been investigating other ways in which the Thunderbolt could be improved. Two P-47D-15-RAs were assigned for testing the Chrysler XI-2220 engine, a 16 cylinder inverted-Vee liquid-cooled unit which transmitted its power to a propeller shaft by way of gears located midway along the crankshaft. The design was such as to produce an extremely finely-streamlined cowling of low frontal area, despite the engine’s ability to produce 2,500 hp. The converted aircraft were designated Republic XP-47H and remembered to a pre-war project, the Republic XP-69. Extensive redesign of the P-47 airframe was necessary to install the XI-2220-11 engine, with which was associated a General Electric CH-5 single-stage turbosupercharger in a modified installation in the rear fuselage. The first flight was not made until July 1945 and in one of the 27 flights a top speed of 414 mph had been recorded. The second XP-47H flew briefly after the war ended (Ref.: 9).

COMMENT: In 1942 studies made by Republic culminated in proposals for a “lightweight” P-47 with an improved engine installation. Construction of two prototypes was authorized, but within a few weeks it became clear to Republic and the USAAF that the more radical development of the P-47 powered by a 3,000 hp Pratt & Whitney R-4300 Wasp Major, than proceeding as the Republic P-72 held greater promise than the P-47J within a similar timescale, and work on the latter project was limited to a single XP-47J. This sole prototype had a lightened wing structure, a more powerful variant of the R-2800 radial engine, driving a four-bladed propeller with a large spinner and a fan to assist the flow of cooling air through a narrow annulus around the spinner. A separate intake scoop beneath and behind the engine cowling provided air for the General Electric CH-5 supercharger in the rear fuselage. The first flight was made in November 1943, and on August 1944 a speed of 505 mph was recorded, at 34,450 ft. This was the highest known speed achieved up to that time in level flight by a propeller driven aircraft and established the XP-47J as one of the few such aircraft to have broken through the 500-mph “barrier” (Lit.: 9).

POWER PLANT: Three × General Electric J-47-GE-13 turbojet engines, rated at 2,700 kp each

PERFORMANCE: 645 mph

COMMENT: In early 1945, USAAF issued requirement for a new attack bomber for low-level bombing and close support as a successor to the Douglas A-26 Invader. Martin Company proposed its design and won the competition with designation, XA-45. Soon later USAAF revised its requirement for better close-support bombing. Martin accepted the new requirement and was received contract for two prototypes, the project was redesignated XB-51. The first XB-51 made its first flight on Oct 1949. The aircraft was powered by three jet engines: one at the extreme tail with an intake at the base of the tailfin, and two underneath the forward fuselage in pods. The innovative, variable incidence wings were swept at 35° and with 6° anhedral. The main landing gear consisted of dual sets of wheels in tandem in the fuselage with outrigger wheels at the wingtips. Crew provision was for a pilot under a “fighter”-type bubble canopy and for an operator/navigator in a compartment located lower than and to the rear of the cockpit. It became the fastest ground support bomber at the time. Although test flights were satisfying Martin XB-51 never went into production. Noteworthy is the fact that the design can be traced back to a German WWII-project Messerschmitt Me P.1102/105 that was to be powered by three Heinkel-Hirth HeS-109-011 turbo-engines, one in the extreme tail and two in pods under the extreme forward fuselage and provided with variable-sweep wings, too (Ref.: 24)

COMMENT: In 1940 the Curtiss Aircraft Company proposed a new design for the eventual replacement for the Curtiss P-40. The new aircraft, the Curtiss XP-60, went through a long series of prototype versions with different engines. Installation of the British Rolls-Royce Merlin engines led to the development of the XP-53, soon redesignated XP-60. Delayed deliveries of the Merlin engines necessitated the installation of Wright (XP-63C) as well as Chrysler (XP-60B) engines. To improve the performance of the XP-60C the engine was changed again, this time a Pratt & Whitney R-2800 with contra-rotating propellers was installed. During its first flight the results were generally satisfactory. Changing the contra-props into a four-bladed propeller led to the XP-60E. But the performance was poorly, too, and further work on this design was abandonded. (Ref.: 8)

COMMENT: The Beechcraft XA-38 Grizzly was a ground attack aircraft, fitted with a forward-firing 75 mm cannon with 20 rounds mounted in a fixed position on the nose to attack heavily armored targets such as tanks and bunkers. Additionally, there were two forward firing machine guns. Defensive armament consisted of remotely controlled ventral and dorsal turrets directed by periscope sights. The first prototype flew on 7 May 1944 and the aircraft proved satisfactory in all respects and better than expected. But after testing it became obvious it would not be ready for the projected invasion of Japan, and furthermore it used the same engines required by the Boeing B-29 Superfortress which had priority. So no orders were given and the XA-38 program was canceled after two prototypes had been completed. (Ref.: 23)